My guess is mIRC received a QUIT but the socket is still active; mIRC still thinks it's in the process of disconnecting.
As starbucks_mafia said, mIRC is designed to connect to IRC servers. When a QUIT message is received the only other valid operation according to the protocol is a closed socket-- initiating another connection on a socket that was flagged as disconnecting is not valid in the context of IRC. This would be expected behaviour of any client that implements the proper IRC protocol, and not a bug.
That said, it seems trivial for mIRC to allow for mIRC to be able to restart a connection in a "disconnecting" state. There is precedent for mIRC breaking pure protocol implementation for popular functionality, and I don't see any compatibility issues with such a change. But this would be a new feature, not a bug fix.